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Comprehensive Understanding of Reduction Mechanisms of Ethylene Sulfite in EC-Based Lithium-Ion Batteries Fucheng Ren,† Wenhua Zuo,‡ Xuerui Yang,† Min Lin,‡ Liangfan Xu,‡ Wengao Zhao,† Shiyao Zheng,‡
and Yong Yang*,†,‡
†College of Energy and ‡State Key Laboratory for Physical Chemistry of Solid Surfaces and Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, China
*S Supporting Information
ABSTRACT: The reliable electrolyte additives are critically important to satisfy the rapidly increasing demand for electrical energy storage with safety, low cost, long life, and high-energy/ power density. As an effective electrolyte additive, ethylene sulfite (ES) is widely used in lithium-ion batteries to improve their cycling performance at low temperature. However, its working mechanism, particularly in ethylene carbonate (EC)- based electrolyte, is still elusive. Here, we present a comprehensive theoretical study of the reduction mechanism of ES in EC-based electrolyte with the ring-opening reaction followed by dimerization reaction and/or the second-electron reduction path. The effects of participation of cosolvents such as diethyl carbonate (DMC), ethyl methyl carbonate (EMC), and dimethyl sulfite (DMS) on the ring-opening reaction process of ES were also carefully investigated. Based on our calculation results, the reactivity order of the clusters (ES)Li+(M) (M = EC, PC, VC, DMC, DMS, and EMC) is shown as follows: (ES)Li+(VC) (4.09 × 1055 s−1) > (ES)Li+(PC) (9.21 × 1047 s−1) > Li+(ES) (8.47 × 1047 s−1) > (ES)Li+(EC) (7.07 × 1047 s−1) > (ES)Li+(DMS) (3.11 × 1043 s−1) > (ES)Li+(EMC) (1.91 × 1041 s−1) > (ES)Li+(DMC) (2.48 × 1037 s−1). The implication of our calculation results on the formation of SEI on the graphite is also discussed.
Lithium-ion batteries (LIBs) offer various advantages com- pared to other rechargeable electrical energy storage systems;1−4 thus, they have been widely applied in our daily lives such as portable electronics and electric vehicles.5−7
However, with the rapid development of our society and technology, there comes a great demand for advanced LIBs with longer cycling life, higher safety, and higher-energy/power densities. During the working and storage of LIBs, a Li+
conducted, whereas electron-insulated layer called solid electrolyte interphase (SEI) was formed on the electrode surface by the complex reactions of the electrolytes. By separating electrodes from direct contact with an electrolyte, the SEI is effective in preventing further electrolyte decomposition and ensuring continual Li+ intercalation/ deintercalation.8 As a result, the properties of SEI such as composition, uniformity, and stability directly correlate to the cycling performance and calendar life of LIBs.9,10
Currently, EC is the most widely used electrolyte solvent in LIBs, not only because of its ability to dissolve and dissociate lithium salt but also more importantly because of its functionality to decompose and form an SEI layer on the electrode surface to protect the electrode from exfoliation in the initial charge process.11 However, the SEI formed by EC tends to be inhomogeneous and loose, which cannot provide a
good long-term protection for electrode materials. Electrolyte additives are commonly used as a simple and effective approach to form a homogeneous SEI with proper thickness and high Li+ conductivity and therefore to prevent or alleviate unwanted parasitic reactions.9,10 Organic sulfur-containing compounds such as sulfites, sulfates, and sultones are generally soluble in electrolyte solvents and have been proposed as SEI- forming additives with promising performances.12−15 The reduction potential of these additives is normally higher than that of typical solvents such as EC, PC, and DMC, and the reduction products may get incorporated into the SEI film and thus lead to the improvements of cell performance.9 Among the sulfur-containing compounds listed above, ethylene sulfite (ES) attracted enormous amounts of attention as an outstanding film-forming additive.16−21 Dahn et al.22−24
found that the introduction of ES (1 wt %) to an EC-based electrolyte could effectively improve the cycling stability of NMC/graphite pouch cells. Ota et al.18 suggested that the SEI film on graphite anode contains both inorganic materials like Li2SO3 and organic materials like ROSO2Li when ES is used as an electrolyte additive.
Received: December 13, 2018 Revised: February 1, 2019 Published: February 22, 2019
pubs.acs.org/JPCCCite This: J. Phys. Chem. C 2019, 123, 5871−5880
© 2019 American Chemical Society 5871 DOI: 10.1021/acs.jpcc.8b12000 J. Phys. Chem. C 2019, 123, 5871−5880
us t 1
, 2 01
pubs.acs.org/JPCC http://pubs.acs.org/action/showCitFormats?doi=10.1021/acs.jpcc.8b12000 http://dx.doi.org/10.1021/acs.jpcc.8b12000
To dive deeper into the film-forming mechanisms of ES, several theoretical studies have been carried out. First, Xing et al.25 proposed a one-electron reduction mechanism in the gas phase. After that, Leggesse et al.26 studied the two-electron reduction mechanism of ES in PC-based electrolyte in vacuum and solvent. Recently, Sun et al.27 built supermolecular clusters [(ES)Li+(PC)m](PC)n (m = 1−2; n = 0, 6, and 9) to investigate the effect of explicit solvent molecules on the ring- opening step of ES and found that the theoretical reduction potential of ES is in agreement with the experimental one. However, previous studies only considered the effect of PC molecule while ignoring the effect of EC and cosolvents such as DEC, DMC, and DMS on the ES reduction process. Furthermore, as an essential part of the SEI film formation process, the termination reactions of the radical anions formed by ES in electrolyte solvents are seldom reported before. As a result, the reduction mechanisms and the possible termination reactions of ES need further exploration and perfection to help further understand the components of SEI in depth. In this paper, we present a comprehensive theoretical
calculation of ES in EC solvents with the ring-opening reaction followed by dimerization reaction and/or the second-electron reduction path. It is theoretically confirmed that ES can be reduced prior to EC to form a reduction precursor. The effects of the implicit and explicit solvents on the reduction reaction of ES in EC-based electrolyte were systematically studied. Especially, the clusters (ES)Li+(M)n (M: EC, n = 1−3; PC, VC, DMC, EMC, DMS, n = 1) were built to investigate the effect of explicit molecules on the ring-opening reaction of ES. The results show that the electron affinity of the clusters monotonously decreases with the number of EC molecule increasing from 1 to 3, while it can be facilitated by cooperating with linear molecules and VC. Linear electrolyte molecules cannot produce an effective SEI film by themselves and inhibit the formation of SEI by increasing the ring-opening barrier of ES. The ring-opening rate constant was also calculated to describe the reactivity of the clusters. Termination reactions followed by the radical anion of Li+(ES)− and (ES)−Li+(EC) are systematically analyzed, which is able to synergistically yield the SEI layer on graphite electrodes.
2. COMPUTATIONAL DETAILS
All of the calculations were carried out using the Gaussian 09 package.28 The equilibrium and transition structures were fully optimized by the B3PW91 method,29−32 using the triple split valence basis set 6-311G along with a set of d, p polarization functions on heavy atoms and hydrogen atoms.33 To confirm the transition states and make zero-point energy (ZPE) corrections, frequency analyses were performed at the same level. Intrinsic reaction coordinate (IRC) calculations were done to confirm whether the transition states correctly connect the stationary points. Both the explicit and implicit solvent effects were considered. The implicit solvent effect was accounted for by using the polarized continuum model (PCM)34 as implemented in Gaussian 09, in which a conventional set of Pauling radii is used for all calculations. A dielectric constant of EC (89.6) was used for all PCM calculations. The specific solvent molecules were explicitly included in the calculations using the clusters (ES)Li+(M)n (M: EC, n = 1−3; PC, VC, DMC, EMC, DMS, n = 1).
3. RESULTS AND DISCUSSION 3.1. Reductive Activity of ES, EC, Li+(ES), Li+(EC), and
(ES)Li+(EC). The calculated binding energy (ΔEb), vertical electron affinity (ΔE), and frontier molecular orbital energy (LUMO) of ES and EC are shown in Figure 1. Both ES and
EC can be associated with lithium ion by the oxygen of sulfo and carbonyl groups, spontaneously. The lowest ΔEb of Li+(EC) (−47.32 kcal/mol) indicates that it is the most stable association form among the calculated compounds, followed by (ES)Li+(EC) and Li+(ES), which is consistent with the bond length increasing trend of Li+-ES (1.971 Å) < Li+-EC (1.869 Å) (shown in Figure 2). The lowest unoccupied
molecular orbital (LUMO) of the isolated solvent molecule and vertical electron affinity (ΔE) ar